The present invention relates to die-casting machines and in particular to a multiple-slide die-casting machine which includes a mold clamping system and an injection system.
Multiple-slide die-casting machines are known in the prior art, and they have at least two mold sections carried by shanks that are driven towards and away from each other. Molten metal is injected into the cavity formed between the two mold sections when the two mold sections are in a molding position and restrained together in a preloaded state. An example is described in applicant""s U.S. Pat. No. 4,601,323, issued on Jul. 22, 1986. A machine for injection molding or die-casting according to that patent includes a main machine base with an injection unit mounted on the rear face and a mold guideway mounted on the front face. An aperture in the main machine base and a corresponding one in the base of the guideway provide for the nozzle of the injection unit to engage molds carried in the guideway. The reciprocating of the mold sections towards and away from one another is due to the action of a toggle assembly interconnecting mold carrying shanks with compression lever brackets mounted in the ends of the guideway, actuators located centrally of guideway ends and linked to the toggle assembly. Position adjusters are used for adjusting the location of the injection unit on the rear of the machine to position its nozzle relative to the molds.
It is important that the contact surfaces of the mold sections do not move, because they constitute the reference plane of the whole mold assembly. The contact plane is called the main parting line. However, in a preloaded state which is required to prevent the two mold sections from moving back while the pressurized melting metal is injected into the cavity between the mold sections, all the components of the clamping assemblies are stressed by the clamping force. The clamping force causes the table and the brackets which support the clamping assemblies to deflect because in a standard machine the said brackets are outrigged over the base. The pre-load force has to be higher than the reaction force induced by the injection pressure. Therefore, the deflection of the table and brackets caused by the clamping force is not to be ignored, and induces deformation of the mold guiding system which causes a misalignment of mold sections. Excessive wear of the slides and poor quality of molded product, such as flash formed along the parting line of the molded product, result from the base deflection and bracket deflection and mold mismatch.
There is a need for improvement of the structure of the machine to inhibit the deflection of the base in the preloaded state.
Study shows, nevertheless, the base deflection, bracket deflection and mold mismatch induced by the clamping force are not the only reason to produce the flash on the molded product. Hot chamber die-casting machines have traditionally been equipped with open loop control injection system. A key feature of the open loop control is that the pressure and flow rate of the hydraulic fluid supplied to the injection cylinder cannot be varied during the injection cycle. These parameters can be changed, but are fixed for any given injection cycle.
At the start of the cycle, hydraulic fluid fed to the injection cylinder causes the injection plunger to accelerate rapidly, then travel at approximately constant velocity to fill the cavity between the mold sections with melting metal. Once the cavity of the mold and runner system have been filled, all the moving components of the injection system come to a sudden stop. This results in a rapid increase in metal pressure within the cavity of the mold, called the xe2x80x9chammer effectxe2x80x9d which often causes flash on the products. Although the degree of control over the injection process is somewhat limited with an open loop system, it is satisfactory for many applications.
For the past several years, closed loop control of the injection systems has been possible. Examples are described in U.S. Pat. No. 4,660,620, issued to Ozeki on Apr. 28, 1987, and U.S. Pat. No. 5,988,260 issued to Iwamoto et al. on Nov. 23, 1999.
Generally, the pressure and flow rate of the hydraulic fluid supplied to the injection cylinder in a closed loop control are changed during the injection cycle, and follow predetermined velocity and/or pressure profiles, and therefore the injection of the molten metal to the cavity of the mold is controlled in an optimum manner. However, the closed loop control of the injection system is currently used with large, conventional die-casting machines which have a relatively long injection time. That is because the system needs a certain minimum stroke to be able to react on and profile the injection. If a product (cast part) has to be molded which is smaller than one requiring the minimal stroke, it is typical to have to change a gooseneck of the injection system to install a smaller diameter sleeve and plunger which require a longer stroke to fill the same cavity of the mold. This is not an easy task. A small product can be produced in a very simple manner if the injection system of the machine can be switched from closed loop control to open loop control.
Therefore, there is a need for a multiple-slide die-casting machine which is adapted to change mold control mode easily from a closed loop control to an open loop control for different size products to be molded on the machine.
An object of the present invention is to provide a multiple-slide die-casting machine which is adapted to produce high quality molded products and eliminate or minimize flash on the products.
Another object of the present invention is to provide a multiple-slide die-casting machine having an improved mechanical structure to inhibit deflection of a base of the machine induced by pressure of clamping assemblies for restraining the slide mold sections together in a preloaded state.
A further object of the present invention is to provide a multiple-slide die-casting machine having a control system which is adapted to be selectable for a closed loop control or open loop control for controlling an injection system of the machine to produce molded products requiring different injection strokes.
It is yet a further object of the present invention to provide a control system for an injection system of a multiple-slide die-casting machine which has a simple structure to include both closed loop mold control and open loop mold control, and which is user-friendly to switch between the two control modes.
A still further object of the present invention is to provide a method for operating an injection system of a multiple-slide die-casting machine in selective control modes to produce molded products requiring different injection strokes to ensure the quality of the products.
According to a broad aspect of the present invention, there is provided a multiple-slide die-casting machine including a base, at least a guideway having side walls and two opposed ends mounted on the base, at least two clamping assemblies guided within the respective ends of the guideway for advancing and retracting slide mold sections towards and away from each other, and an injection system for introducing pressurized casting material into a cavity between the slide mold sections when slide mold sections are in a molding position in which the slide mold sections are restrained together in a preloaded state, the multiple-slide die-casting machine comprising at least two brackets supported on the base for operatively securing the respective clamping assemblies, and a reinforcement means for interconnecting the brackets to inhibit deflection of the base and the brackets induced by the force generated by the clamping assemblies for maintaining the preloaded state.
More especially, according to a further broad aspect of the present invention, there is provided a multiple-slide die-casting machine which has a base plate and a guide member mounted on the base plate. The guide member defines two guideways crossing and perpendicular to each other, each guideway having side walls and two opposed ends. A respective clamping assembly is guided within each of the ends of each guideway for advancing and retracting a slide mold section towards and away from a centre of the guideway. An injection system is provided for introducing pressurized casting material into a cavity between the slide mold sections when the slide mold sections are in a molding position in which the slide mold sections are restrained together in a preloaded state. A respective bracket including a first surface secured to the base plate and a second surface remote from the base plate operatively secures each of the clamping assemblies between the first and second surfaces thereof. Interconnection means interconnects the second surfaces of the brackets so that the respective clamping assemblies are operatively secured between the base plate and the interconnection means, and deflection of the base plate and the brackets induced by the force generated by the clamping assemblies for maintaining the preloaded state is inhibited.
Each of the clamping assemblies preferably comprises a clamping mechanism and a shank having opposed ends. The shank is slidable between the side walls in one of the ends of one guideway, connected at a first end thereof to one of the slide molds and coupled at a second end thereof to the clamping mechanism. The shank is coupled to the clamping mechanism through a ram and a coupling. A respective pair of stops preferably provided between each of the brackets and each of the rams to ensure the precise molding position of the slide mold sections.
Each of the couplings preferably comprises a plurality of pivotal link members adapted to transfer a translation of the clamping mechanism to a translation of the ram and shank while permitting misalignment of the translations being transferred.
Preferably, each of the clamping mechanisms is adjustably secured to a corresponding one of the brackets to ensure the pressure of the clamping assemblies for maintaining the preloaded state, as predetermined.
In accordance with another aspect of the present invention, a control system for an injection system of a multiple-slide die-casting machine is provided. The injection system has a hydraulic cylinder for advancing and retracting an injection plunger adapted to introduce a pressurized casting material into a cavity between a plurality of slide mold sections. The control system comprises means for detecting a position of the injection plunger, including at least one position transducer for detecting a predetermined position of the injection plunger where hydraulic cylinder control parameters should be changed from a velocity phase in which a velocity of the injection plunger follows predetermined profiles, to a pressure phase in which a net hydraulic pressure applied to the injection plunger is controlled. A servo valve is provided for controlling a flow rate of hydraulic fluid supplied to the hydraulic cylinder. A controller is adapted to selectively control the hydraulic cylinder through the servo valve in a closed loop control mode and an open loop control mode. In the closed loop control mode the controller receives signals from the position detecting means and sends command signals in accordance with the predetermined velocity profiles to actuate the servo valve accordingly in the velocity phase, and sends command signals in accordance with predetermined pressure profiles to actuate the servo valve accordingly in the pressure phase. In the open loop control mode the controller sends a constant command signal to set a pre-selected flow rate on the servo valve for the desired velocity of the injection plunger.
A control mode selection valve is preferably provided to be automatically activated only when the open loop control mode is selected to enable a selected reduced pressure pre-set on a pressure reducing valve. A microprocessor and a user interface are preferably provided for programming the velocity and pressure profiles used in the closed loop control mode, and selecting the desired velocity of the injection plunger in the open loop control mode.
In accordance with yet another aspect of the present invention, there is provided a method for operating the injection system of the multiple-slide die-casting machine. The method comprises the steps of advancing the injection plunger to introduce the casting material into the cavity between the slide mold sections in the closed loop control mode when a regular injection stroke is needed to cast a product; and advancing the injection plunger to introduce the casting material into the cavity between the slide molds in the open loop control mode when an injection stroke is needed to cast a small sized product.
The multiple-slide die-casting machine incorporating the invention advantageously provides flash-free castings of improved surface finish by the use of the full clamping capacity of the clamping system and the selective use of the closed loop control and open loop control for the injection system to meet the different requirements of injection for different size products. Other features and advantages of the invention will be better understood with reference to the preferred embodiments described hereinafter.